1. Technical Field
The present application relates to a method of reducing silicosis caused by inhalation of silica-containing proppant, such as silica sand and resin-coated silica sand, and apparatus therefor.
2. Background Information
Hydraulic fracturing is the propagation of fractures in a rock layer, which process is used by oil and gas companies in order to release petroleum, natural gas, coal seam gas, or other substances for extraction. The hydraulic fracturing technique is known in the oil and gas industry as “fracking” or “hydrofracking.” In hydraulic fracturing, a proppant is used to keep the fractures open, which proppant is often a silica-containing material, such as silica sand and resin-coated silica sand. Many tons of proppant are used at a fracking site, thereby exposing workers to inhalation of silica dust, which can lead to a lung disease known as silicosis, or Potter's rot. Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust, and is marked by inflammation and scarring in forms of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis, or lung disease caused by the inhalation of dust, usually from working in a mining operation.
When preparing proppant for use in hydraulic fracturing, large amounts of dust, such as silica dust and other proppant dust, are created by the movement of proppants. This dust can produce potential detrimental effects, such as contaminating atmospheric air, creating a nuisance to adjacent landowners, and damaging equipment on the hydraulic fracturing site. A significant concern, as discussed above, is the inhalation of silica dust or other proppant dust, which can lead to lung conditions such as silicosis and other specific forms of pneumoconiosis.
Hydraulic fracturing jobs use a large amount of proppant, often as much as 15,000 tons. This large quantity of proppant is brought in by pneumatic tankers and then blown into proppant storage trailers known as “mountain movers,” “sand hogs” or “sand kings.” Some well-known storage devices of this type have been manufactured by Halliburton. These storage trailers have access doors on top which vent the incoming air to the atmosphere. The flow of air creates large dust clouds, such as silica dust clouds, which blow out of the access doors, which can be especially problematic for workers who are looking into the interior of the storage trailers to monitor the proppant fill level. The proppant is then gravity fed onto a conveyor belt that carries the proppant to another conveyor, usually a T-belt which runs transverse to and collects the proppant from multiple storage trailers. The gravity feed of the proppant once again disturbs the proppant resulting in additional dust clouds. The T-belt then carries the proppant to be discharged into the hopper of one or more blenders, at which point the proppant is again disturbed and additional dust clouds are created.
During this entire process, workers are often standing near or directly in the path of a cloud or airborne flow of silica dust or proppant dust. When small silica dust particles are inhaled, they can embed themselves deeply into the tiny alveolar sacs and ducts in the lungs, where oxygen and carbon dioxide gases are exchanged. The lungs cannot clear out the embedded dust by mucous or coughing. Substantial and/or concentrated exposure to silica dust can therefore lead to silicosis.
Some of the signs and/or symptoms of silicosis include: dyspnea (shortness of breath), persistent and sometimes severe cough, fatigue, tachypnea (rapid breathing), loss of appetite and weight loss, chest pain, fever, and gradual dark shallow rifts in nails which can eventually lead to cracks as protein fibers within nail beds are destroyed. Some symptoms of more advanced cases of silicosis could include cyanosis (blue skin), cor pulmonale (right ventricle heart disease), and respiratory insufficiency.
Aside from these troublesome conditions, persons with silicosis are particularly susceptible to a tuberculosis infection known as silicotuberculosis. Pulmonary complications of silicosis also include chronic bronchitis and airflow limitation (similar to that caused by smoking), non-tuberculous Mycobacterium infection, fungal lung infection, compensatory emphysema, and pneumothorax. There is even some data revealing a possible association between silicosis and certain autoimmune diseases, including nephritis, scleroderma, and systemic lupus erythematosus. In 1996, the International Agency for Research on Cancer (IARC) reviewed the medical data and classified crystalline silica as “carcinogenic to humans.”
In all hydraulic fracturing jobs, a wellbore is first drilled into rock formations. A hydraulic fracture is then formed by pumping a fracturing fluid into the wellbore at a rate sufficient to increase pressure downhole to exceed that of the fracture gradient of the rock to be fractured. The rock cracks and the fracture fluid continues farther into the rock, thereby extending the crack or fracture. To keep this fracture open after the fluid injection stops, the solid proppant is added to the fluid. The fracturing fluid is about 95-99% water, with the remaining portion made up of the proppant and chemicals, such as hydrochloric acid, methanol propargyl, polyacrylamide, glutaraldehyde, ethanol, ethylene glycol, alcohol and sodium hydroxide. The propped fracture is permeable enough to allow the flow of formation fluids to the well, which fluids may include gas, oil, salt water, fresh water and fluids introduced during completion of the well during fracturing. The proppant is often a silica-containing material, such as sand, but can be made of different materials, such as ceramic or other particulates. These materials are selected based on the particle size and strength most suitable to handle the pressures and stresses which may occur in the fracture. Some types of commercial proppants are available from Saint-Gobain Proppants, 5300 Gerber Road, Fort Smith, Ark. 72904, USA, as well as from Santrol Proppants, 50 Sugar Creek Center Boulevard, Sugar Land, Tex. 77478, USA.
The most commonly used proppant is silica sand or silicon dioxide (SiO2) sand, known colloquially in the industry as “frac sand.” The frac sand is not just ordinary sand, but rather is chosen based on certain characteristics according to standards developed by the International Organization for Standardization (ISO) or by the American Petroleum Institute (API). The current ISO standard is ISO 13503-2:2006, entitled “Petroleum and natural gas industries—Completion fluids and materials—Part 2: Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations,” while the API standards are API RP-56 and API RP-19C. In general, these standards require that the natural sands must be from high silica (quartz) sandstones or unconsolidated deposits. Other essential requirements are that particles are well rounded, relatively clean of other minerals and impurities and will facilitate the production of fine, medium and coarse grain sands. Frac sand is preferably >99% quartz or silica, and high purity quartz sand deposits are relatively common in the U.S. However, the tight specifications for frac sands—especially in relation to roundness and sphericity—make many natural sand deposits unsuitable for frac sand production. One primary source of such high quality sand is the St. Peter sandstone formation, which spans north-south from Minnesota to Missouri and east-west from Illinois into Nebraska and South Dakota. Sand from this formation is commercially known as Ottawa sand. This sand generally is made of a very high percentage of silica, and some samples, such as found in Missouri, consist of quartz sand that is 99.44% silica.
One characteristic used to determine suitability of a proppant material, such as silica sand, is grain size, which can be measured using standard length measurements or by mesh size. Mesh size is determined by the percentage of particles that are retained by a series of mesh sieves having certain-sized openings. In a mesh size number, the small number is the smallest particle size while the larger number is the largest particle size in that category. The smaller the number, the coarser the grain. The vast majority of grains range from 12 to 140 mesh and include standard sizes such as 12/20, 16/30, 20/40, 30/50, and 40/70, whereby 90% of the product falls between the designated sieve sizes. Some specific examples are 8/12, 10/20, 20/40, and 70/140. Grain size can also be measured in millimeters or micrometers, with some examples being grain size ranges of 2.38-1.68 mm, 2.00-0.84 mm, 0.84-0.42 mm, and 210-105 micrometers.
Another important characteristic of a proppant material, such as silica sand, for hydraulic fracturing is the sphericity and roundness of the grains, that is, how closely the grains conform to a spherical shape and its relative roundness. The grains are assessed by measuring the average radius of the corners over the radius of a maximum inscribed circle. Krumbein and Sloss devised a chart for the visual estimation of sphericity and roundness in 1955, as shown in FIG. 4. The API, for example, recommends sphericity and roundness of 0.6 or larger based on this scale.
An additional characteristic of a proppant material, such as silica sand, is crush resistance, which, as the phrase implies, is the ability of the proppant to resist being crushed by the substantial forces exerted on the proppant after insertion into a fracture. The API requires that silica sand withstand compressive stresses of 4,000 to 6,000 psi before it breaks apart or ruptures. The tested size range is subjected to 4,000 psi for two minutes in a uniaxial compression cylinder. In addition, API specifies that the fines generated by the test should be limited to a maximum of 14% by weight for 20-40 mesh and 16-30 mesh sizes. Maximum fines for the 30-50 mesh size is 10%. Other size fractions have a range of losses from 6% for the 70-40 mesh to 20% for the 6-12 mesh size. According to the anti-crushing strength measured in megapascals (MPa), types of frac sand can possibly be divided, for example, into 52 Mpa, 69 Mpa, 86 Mpa and 103 Mpa three series.
Yet another characteristic of a proppant material, such as silica sand, is solubility. The solubility test measures the loss in weight of a 5 g sample that has been added to a 100 ml solution that is 12 parts hydrochloric acid (HCl) and three parts hydrofluoric acid (HF), and heated at 150° F. (approx. 65.5° C.) in a water bath for 30 minutes. The test is designed to determine the amount of non-quartz minerals present. However, a high silica sandstone or sand deposit and its subsequent processing generally removes most soluble materials (e.g. carbonates, iron coatings, feldspar and mineral cements). The API requires (in weight percent) losses of <2% for the 6-12 mesh size through to the 30-50 mesh size and 3% for the 40-70 mesh through to 70-140 mesh sizes.